Modern audiovisual encoding standards, such as MPEG-1 and MPEG-2, provide means for transporting multiple audio and video components within a single transport stream. Individual and separate audio components are alignable to selected video components. Synchronised multi-channel audio, such as surround sound, are only provided for in terms of a single, pre-mixed surround sound audio component, for example a single Dolby 5.1 audio component. However, there are currently no means provided for individualised multi-channel audio components to be transported in a synchronised form.
In particular, the MPEG-1 and MPEG-2 audio specifications (ISO/IEC 11172-3 and ISO/IEC 13818-3 respectively) describe means of coding and packaging digital audio signals. These include schemes that are specified to support various forms of multi-channel sound that use a single MPEG-2 transport stream component. These provisions are backward compatible with the previous MPEG-1 audio system. In the prior art, it is only by assembling the several audio channels into such a single transport component that it is possible to assure the required synchronisation of the channels. These schemes either require:
[a] the use of surround-sound compression methods (e.g. Dolby 5.1) or
[b] the use of proprietary compression techniques, or
[c] the use of uncompressed audio.
The use of surround-sound compression methods reduces the bit rate required for the multiple channels by exploiting the redundancies that exist between the several channels and also the features of the human auditory system that render certain spatial characteristics of the sound to be undetectable and so may be masked in processing. These complex schemes provide adequate means of dealing with a single coding stage in which only one coding and decoding operation is expected, but they are not ideal for signals that, for practical and operational reasons (e.g. source feeds from a remote location to the central editing facilities), need to be re-encoded perhaps several times in transmission networks. This is due to concatenation issues resultant from multiple coding operations in sequence degrading the audio quality. This is particularly the case where capacity is limited, causing the bit rate to be reduced substantially, leaving little headroom to deal with such degradations in concatenated coding and transmission.
The use of proprietary compression techniques typically require the use of additional external proprietary equipment leading to greater expense and operational complication. This method may also suffer the same quality degradation that concatenation of more than one coding/decoding stage produces.
Whereas, if the audio is sent in uncompressed format (e.g. uncompressed Linear PCM samples), the required data rate is very high data rate (e.g. approx 3 Mbit/s per two-channel pair).
Whilst the above is not generally a problem when providing finalised audiovisual media to consumers, it does present a problem for the audiovisual media production industry, because the industry is increasingly taking advantage of ubiquitous modern high speed data networks to send “raw” audiovisual media (i.e. the source material used to produce television, films and other media) instantaneously in compressed form between production facilities, or indeed from the production facilities out to the television or audio network distribution points, e.g. Terrestrial transmitters, Satellite uplinks or Cable head ends.
For example, location camera crews typically feed audiovisual material to central television studios, for editing and distribution to affiliated television stations for eventual broadcast to viewers. The aforementioned audiovisual encoding standards do not allow synchronised multichannel audio to be sent without pre-mixing, hence adding to the complexity of their field equipment, or preventing them from providing multi-channel audio.
There is a particular need to be able to transmit multi-channel audio that has a requirement for accurate channel-to-channel alignment, such that the audio signals can be subsequently encoded as surround-sound audio where the temporal alignment of multiple channels is important, using the above MPEG standards since a majority of production equipment is already set up for use with these standards.
Accordingly, the present invention proposes methods and apparatus that provide a cost-effective and convenient mechanism for delivering multiple channel audio whilst maintaining sound quality and accurate temporal alignment among the channels.